1. Field of the Invention
The present invention relates to an optical module, in particular to an L-band light source.
2. Description of the Related Art
A light source with a wide wavelength band is needed to measure the optical characteristics employed in optical communication. Moreover, the wavelength band of the optical signals used in optical communication is 1520 nm˜1620 nm when at least one erbium doped fiber amplifier (EDFA) is employed. Thus, a light source capable of measuring optical characteristics of various optical components within such a wavelength band is needed.
A wavelength division multiplexing passive optical network (WDM-PON) has recently been highlighted as a technology for a high-speed fiber-to-the-home (FTTH) network. In a WDM-PON attention is paid to the broadband light source that is used along with a wavelength locked Fabry Perot laser diode (FP-LD) in order to accommodate a plurality of subscribers. Existing available broadband light sources mainly employ a white light source or an EDFA outputting amplified spontaneous emission (ASE). However, because white light sources have low output power, they are limited in measuring the optical characteristics of a light source or an optical component for a WDM-PON which requires high output power. In addition, EDFAs are not economical in price.
U.S. Pat. No. 6,507,429 issued to Gaelle Ales et al. and entitled “Article Comprising a High Power/Broad Spectrum Superfluorescent Fiber Radiation Source” discloses a broadband source for outputting C-band (1520 nm˜1570 nm) ASE and L-band (1570 nm˜1620 nm) ASE. The broadband light source includes first and second rare earth element doped optical fibers, and an isolator located between the optical fibers. First pumping light from a first pump light source is supplied to the first rare earth element doped optical fiber and second pumping light from second pump light source is supplied to the second rare earth element doped optical fiber. The first rare earth element doped optical fiber has a length longer than that of the second rare earth element doped optical fiber about five times. A reflector reflects ASE inputted from the first rare earth element doped optical fiber, thus assisting generation of L-band ASE in the first rare earth element doped optical fiber. The second rare earth element doped optical fiber conducts functions of amplifying the L-band ASE and generating C-band ASE. As a result, the broadband light source is able to output C-band and L-band ASEs through an output end thereof.
However, the typical broadband optical source has poor output efficiency. This is due to the isolator being between the first and second rare earth element doped optical fibers; thus, the C-band ASE outputted to the rear side of the second rare earth element doped optical fiber cannot be used. In addition, if the output power of the first pump light source is changed so as to tune the output power of the L-band ASE (obtained from the first rare earth element doped optical fiber), not only the output power of the L-band ASE but also the output power of the C-band ASE is changed. In contrast, if the output power of the second pump light source is changed so as to tune the C-band ASE (obtained from the second rare earth element doped optical fiber), not only the output power of the C-band ASE, but also the output power of the L-band ASE is changed. Accordingly, since the output powers of the C-band ASE and L-band ASE are affected by one another, it is more difficult to control the output power of the broadband light source.